The xeroprinting process employs a printing plate, commonly referred to as a "master", made by creating a pattern of insulating material (i.e., an image) on the surface of a grounded conductive substrate. In the xeroprinting process, the master is exposed to an electrostatic field (e.g., by a corona discharge) that imposes an electrostatic charge on the surface of the master. The portion of the master bearing the insulating material retains the charge, while the charge on the remainder of the master is discharged through the grounded conductive substrate. Thus, a latent image of electrostatic charge is formed on the insulating material, the image subsequently being developed with oppositely charged particles commonly referred to as "toner". The toner is then transferred (e.g., by electrostatic or other means) to another surface (e.g., paper or polymeric film), where it is fused (i.e., "fixed"), to reproduce the image of the master. Since the image on the master is permanent, or at least persistent, multiple copies can be made by repeating the charging, toning and transfer steps.
Recently issued U.S. Pat. No. 4,732,831 to Riesenfeld et al. discloses an improved xeroprinting process that employs a master having a photopolymerizable coating on a conducting substrate. The coating contains an organic polymeric binder, an ethylenically unsaturated monomer, and a photoinitiator system. When the master is exposed to the desired pattern of actinic radiation (i.e., light of a suitable wavelength), exposed regions of the coating polymerize and exhibit a significantly higher electrical resistance than unexposed regions. Thus, when the master is subsequently used in the xeroprinting process, the polymerized regions will tend to hold an electrical charge, which is developed with toner, while the unpolymerized regions discharge to ground through the conductive backing and therefore do not attract the toner.
The electrostatic master of U.S. Pat. No. 4,732,831 offers a number of advantages over the prior art in that there is no development step required between creation of an image on the master and subsequent use of the master in the xeroprinting process. Although the master is well suited for many applications, however, the decay rate for unpolymerized regions is not sufficiently rapid to permit use of the master in a high speed xeroprinting process where the master will rapidly proceed through charging and toning stations. In such processes it is desired that the charge on grounded portions of the master decay to a level that will not attract toner within two (2) seconds or less after exposure to the corona discharge. Otherwise, toner may be carried over on regions of the master that are not sufficiently discharged, adversely effecting quality of the copies. Thus, there is a need for an improved master particularly suited for high speed xeroprinting.